Devices utilizing the Hall effect have been known for many years. Such devices generally comprise a body formed from a material exhibiting the Hall effect, such as indium antimonide (InSb), indium arsenide (InAs), gallium arsenide (GaAs), or other well known materials. Such devices are formed having a rectangular, square or cross shape. The first pair of electrodes are disposed on opposite edges of the body to which is applied a so-called excitation current or voltage. Generally, two additional electrodes are provided opposite each other on the remaining edges of the body. These electrodes are variously called the output, sensor or Hall voltage electrodes.
As is well known, when a Hall device is inserted into a magnetic field having a component normal to the plane of the body of the material, a voltage will be developed at the output electrodes which is proportional to the excitation current and the magnetic field strength.
Devices exhibiting the Hall effect have found use in many areas, including use as switching devices, magnetic sensors, and as an electrical power sensor. In this latter case, a Hall effect device can be used to measure the power flowing in an electrical circuit, such as is used to supply electricity to a home or office. This is done by applying a excitation voltage to the Hall device which is proportional to the AC line voltage and by coupling one or more of the live power carrying lines to a magnetic circuit associated with the Hall effect device so that the strength of the magnetic field being applied to the Hall device is proportional to the current flowing in the measured live lines. The voltage appearing at the output terminals of the Hall effect device will be directly proportional to the product of the instantaneous line voltage times the instantaneous line current, i.e. power or watts. If this power-representative voltage is applied to a voltage-to-frequency converter the output of the converter will be a series of pulses whose frequency is proportional to power. When these pulses are accumulated over time, e.g. by a counter, the resultant accumulated amount of pulses is representative of energy or watt-hours.
One problem associated with prior Hall effect devices is a condition in which an output appears at the output electrodes of the Hall device even though no external magnetic field is present. Ideally, if no magnetic field were present and the Hall device were perfectly symmetrical, the output of a Hall device at its output electrode would be zero, even when an excitation voltage is applied to the excitation electrodes. However, in practice, because of inhomogeneities in the Hall material, manufacturing tolerances and changes in environmental factors, such as temperature and the like, a non-zero output voltage may be present even in the absence of an applied external magnetic field.
In a D.C. device, voltage offsets due to device asymmetry or materials inhomogeneity can be corrected by offsetting the Hall voltage output electrodes so as to lie along the equipotential line closest to the geometrical center of the device. However, such a solution cannot be used in a device where an A.C. excitation voltage is used, such as would be necessary in measuring electrical power or energy in an electricity meter. This is because the position of the equipotential line constantly shifts as the sign of the applied A.C. voltage changes.
It has been found that the voltage offset encountered in A.C. type Hall devices is really composed of two offsets. One offset is a first order (linear) offset due to inhomogeneities in the material comprising the Hall device. This offset term can be readily compensated for by the use of external balancing resistors connected to the output electrodes of the Hall device. However, the other component of the voltage offset is a second order and non-linear function which is believed to be caused by alignment errors in the fabrication of the Hall device. Such alignment errors cause the Hall device to be formed having a somewhat asymmetrical shape. Other causes of this second order offset voltage are also believed to include back-gating and self-gating effects due to electron tunneling in the material as well as non uniformity in the material.
This second order voltage offset cannot be readily corrected using external balancing resistors as can the first order offset term. The presence of this second order voltage offset term in currently available Hall devices makes them unsuitable for use in the measurement of electrical power (e.g. as a power sensor for an electricity meter) because of the high accuracy (e.g. .+-.1/2 percent of measurement) required over the entire measuring range encountered by such devices. Since electricity meters must be capable of accurately measuring power over values ranging from fractions of a watt up to tens of thousands of watts, there is a strong need for a Hall effect device which will provide the necessary accuracy so that such devices can be used as solid state power sensors.